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Protein Synthesis > Post Transcriptional Modifications > Flashcards

Post Transcriptional Modifications Flashcards

1
Q

Which two RNA’s are modified similarly in prokaryotes and eukaryotes and in what way

A
  • tRNA and rRNA

- they are cleaved by ribonucleases

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2
Q

How does prokaryotic mRNA look like

A

Similar to initial transcript

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3
Q

Modifications of rRNA

A
  • pre-mRNA is cleaved by Rnases to give intermediate pieces of rRNA
  • the cleaved pieces are further trimmed by exonuclease to desired shape
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4
Q

What is pre-rRNA

A
  • long precursor molecules for rRNA

- initial transcript for rRNA formation

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5
Q

Sizes of prokaryotic rRNA

A

-23 , 16 , 5S

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6
Q

Sizes of eukaryotic rRNA and polymerase which makes them

A

Pol 2 - 28 ,18 ,5.8S

Pol 3 - 5S

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7
Q

Where does rRNA synthesis occur

A

-in the nucleolus

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8
Q

Modifications of tRNA

A
  • 16 nucleotides sequence at 5’ end is cleaved by RNase P
  • 14 nucleotides sequence at anticodon ( intron ) is removed by nucleases
  • Uracil residues at 3’ end replaced by CCA
  • many bases converted to characteristic bases of tRNA
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9
Q

What enzyme adds CCA sequence to 3’ end of tRNA

A

-nucleotidytransferase

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10
Q

What is an intron and exon

A
  • nucleotide sequence on RNA which does not code for any protein
  • nucleotide sequences on DNA and mRNA which code for proteins
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11
Q

What happens to introns and Exons pre-mRNA

A
  • introns are removed by splicing and not part of mature RNA

- Exons are joined covalently by splicing and make up mature mRNA

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12
Q

4 types of introns and how they are removed

A

1 introns in tRNA - removed by protein

2 introns in protein coding genes - spliceosome

3 self-splicing introns ( Group 1 ) - catalyses their own removal using GTP or other nucleotide cofactors

4 self-splicing introns ( Group 2 ) - do not need GTP to remove them selves

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13
Q

What is hnRNA

A
  • the collection of all transcripts from pol 2

- all pre-mRNA collectively

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14
Q

What is the first mRNA modification and when does it occur

A
  • addition of cap

- added during mRNA transcription

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15
Q

What if the cap

A

-7-methyguanosine triphosphate

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16
Q

What type of bond is formed by mRNA cap and use

A
  • a 5’-5’ phosphodiester bond
  • linkage resistant to nucleases
  • guides mRNA movement out of nucleus
  • stabilizes mRNA
  • used for recognition by ribosomes
  • permits efficient initiation of translation
17
Q

How is 7-methy cap formed

A
  • 1st a phosphoryl group is removed from 5’ triphosphate of pre-mRNA
  • guanosine mono phosphate is then added and terminal guanosine methylated
18
Q

Enzyme for adding guanosine monophosphate , for methylation of guanosine and source of methyl and location of methylation

A
  • guanylyltransferase
  • guanine-7-methyltransferase
  • s-adenosylmethionine
  • in Cytosol
19
Q

When is poly-A tail added and function

A

-post transcriptionally

  • stabilizes mRNA
  • facilitates exist into cytosol
20
Q

Addition of poly-A tail steps

A
  • pre-mRNA is bound at specific sequences by cleavage factors at 3’ end
  • the factors align 3’ end in correct orientation for cleavage
  • stabilizing factors are added to cleavage complex
  • polyadenylate polymerase binds to pre-mRNA and cleaved of 3 end and adds tail using adenosine triphosphates as substrates
21
Q

Length of tail and name of signal sequence

A
  • 40-250 adenine nucleotides

- polyadenylation signal sequence

22
Q

When Removal of introns occur

A

-during maturation of mRNA before it enters cytosol to go to ribosomes

23
Q

What is splicing

A

-removal of introns and covalently joining together of Exons

24
Q

What is a spliceosome

A

-molecular complex with snRNP

/has a lariat structure

25
Q

Role of snRNA

A
  • they associate with proteins to form snRNP that mediate splicing
  • they facilitate removal of introns by binding with introns at specific consensus sequences
26
Q

The 6 snRNP and their functions

A

1 U1 - binds the 5’ donor site

U2 - bind the branch A site

U3 to U6 complete the complex

27
Q

Mechanism of splicing

A
  • binding of snRNP brings Exons closer together
  • a 2-OH end of a adenine phosphate within intron ( branch A site ) attacks 5’ end of introns ( splice donor site )
  • forms 2’-5’ phosphodiester bond creating a lariat structure
  • newly freed 3-OH of exon attacks 5’ end of exon 2 at splice acceptor site forming 5’-3’ phosphodiester bond
28
Q

What happens to spliced intron lariat

A

/degraded and nucleotides recycled

29
Q

Effect of splice site mutations

A

-leads to added intron or delete exon

/results in defected protein synthesized

30
Q

Alternate splicing use

A

-produces variations in mRNA therefore altering protein synthesized
/ is a mechanism for producing diverse proteins with a limited set of genes

Protein Synthesis (8 decks)

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